The boat was hijacked just after leaving its navy escort and taken to a creek in the swampy Niger Delta.

It was the second group of foreigners to be kidnapped and then released in the past four days in the Delta, which has seen a recent surge in violence.

Two people were shot and wounded and eight foreign oil workers kidnapped on Saturday.

The eight, also believed to be Russians, were released hours after they were taken from a tanker on the Bonny River.

In both cases, security officials said no ransom had been paid.

More than 200 foreign oil workers have been kidnapped in the Niger Delta over the past two years, but they are often released after payment of ransom.

The group of five released on Monday worked for the oil services company Saipem.

It is believed the tanker which came under attack belongs to Global Gas and Refining Ltd, a Nigerian subsidiary of US-based Global Energy Inc, which has been stationed along the Bonny river for more than two years.

It is not known who carried out the kidnappings.

Mend ended a unilaterally announced ceasefire after British Prime Minister Gordon Brown said the UK would help train the Nigerian military amid efforts to tackle violence in the Delta.

The militants say they are fighting to ensure that Niger Delta residents see more benefits of the oil wealth.

But some are criminal gangs, making money from ransom payments and stealing oil.

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